Given current trends toward higher functionality, higher speeds, smaller sizes and greater integration, the LSI chips such as CPUs, driver ICs and memories that are used in electronic equipment such as personal computers and cell phones themselves generate large amounts of heat. The rise in chip temperature due to such heat causes chip malfunctions and failure. To address this, numerous heat-dissipating methods for suppressing a rise in chip temperature during operation, and heat-dissipating members for use in such methods, have been described.
Recently, in electronic equipment and the like, heat sinks that employ a metal plate having a high thermal conductivity, such as one made of aluminum or copper, are being used to hold down the rise in chip temperature during operation. Such a heat sink carries away heat generated by the chips and discharges the heat from a surface by way of the temperature difference with outside air.
To efficiently carry away heat generated by the chips in an electronic device, it is necessary to place the heat sink in close contact with the chips. Because of height differences among the chips and the existence of tolerances due to assembly work, a sheet having flexibility or a grease is placed between the chips and the heat sink, and heat conduction from the chips to the heat sink is achieved through this sheet or grease.
With grease-type heat-dissipating materials, a low thermal resistance is achieved by rendering the material into a thin film, but control is difficult. In the application step, there are cases where screen printing or dispensing from a syringe is carried out manually and other cases where application is carried out automatically using a dispenser. However, because such application takes a lot of time and handling is not easy, it is sometimes rate-limiting for the product assembly operation.
Thermally conductive sheets formed of a thermally conductive silicone rubber or the like have an excellent handleability compared with greases, and are used in various fields.
In particular, thermally conductive sheets of low hardness have the following advantages: owing to their shape flexibility, they are capable of faithfully following uneven shapes among devices such as CPUs; they do not prevent the miniaturization of portable equipment such as notebook-type personal computers; and they make efficient heat dissipation possible.
Given the trend in recent years toward increased heat output by heat-generating devices as the level of device integration rises, there exists a desire for heat-dissipating sheets that have a low stress and a high thermally conductivity. Generally, to increase the thermal conductivity of the sheet, it is necessary to load the silicone resin with a large amount of thermally conductive filler, but doing so has the undesirable effects of lowering the compressibility and reliability. To address this, examples have been reported in which, for dielectric applications, a filler having a high thermal conductivity, such as boron nitride or aluminum nitride, is used in order to achieve a higher thermal conductivity at lower loadings. However, depending on the boron nitride, the particles may be flake-like in shape. Therefore, when added to a silicone resin, the particles are incorporated in a reclining state, making it difficult to successfully achieve high thermal conductivity in the a-axis direction. Special treatment that causes the boron nitride particles to be incorporated in a standing state is thus necessary.
A variety of heat-dissipating materials in which aluminum nitride has been selected for use as a highly heat-conductive filler have been reported (Patent Document 1: JP-A H03-14873; Patent Document 2: JP-A H03-295863; Patent Document 3: JP-A H06-164174; Patent Document 4: JP-A H11-49958).
However, aluminum nitride is known to generate aluminum hydroxide and ammonia gas via a hydrolysis reaction with moisture. Because aluminum hydroxide has a much lower thermal conductivity than aluminum nitride powder and the ammonia gas remains behind in the heat-dissipating member as gas bubbles, these cause a decline in the heat-dissipating properties of the heat-dissipating member.
Methods have been proposed that use large-size aluminum nitride having an average particle size of about 100 μm and a high resistance to hydrolysis. However, particularly at reduced device thicknesses, surface irregularities arise on account of the coarse particles, which is a problem in that adherence during packaging decreases and the thermal resistance grows larger.
Hence, from the standpoint of adherence to the substrate, a method that resolves the problem by using a ground-up material having an average particle size of 50 μm or less (Patent Document 5: JP-A H6-209057) and a method which uses in combination a ground-up material having an average particle size of not more than 30 to 50 μm obtained from sintered compacts of aluminum nitride and unsintered aluminum nitride having an average particle size of 0.1 to 5 μm (Patent Document 6: JP-A H6-24715) have been disclosed. However, fully enhancing the thermal conductivity has been difficult on account of, in the former, the small particle size, and in the latter, the high content of fine powder.
Also, Patent Document 7 (JP No. 4357064) discloses a heat-dissipating member that has excellent thermal conductivity and resistance to hydrolysis by optimizing the particle size makeup of the aluminum nitride. This is characterized in that the thermal conductivity and adherence are both increased by concomitantly using aluminum nitride having an average particle size of 1 to 3 μm. However, with aluminum nitride having an average particle size of 1 to 3 μm, there is a concern that the material will end up having a higher viscosity and a lower moldability. Moreover, selecting aluminum nitride as the fine powder leads to higher costs.
Patent Document 8 (JP-A 2004-91743 discloses a thermally conductive grease which, by including spherical alumina having an average particle size of 0.2 to 1.0 μm and aluminum nitride having an average particle size of 1 to 3 μm and a maximum particle size of 2 to 10 μm, suppresses an increase in viscosity, enabling a thinner film to be obtained. However, a drawback is that, because the particle size of the aluminum nitride is small, achieving a higher thermal conductivity is difficult.
In silicone-based heat-dissipating members that use aluminum nitride, the challenge is to provide heat-dissipating members having a high thermal conductivity and excellent water resistance. By including large-particle-size aluminum nitride, the water resistance and thermal conductivity improve, but surface irregularities readily arise and the thermal resistance increases. Although one conceivable approach is to increase adherence by including a fine powder such as aluminum nitride, an increased amount of fine powder is an obstacle to achieving a higher thermal conductivity. A formulation which suppresses a decline in the thermal conductivity and can increase adherence is thus required. In the interest of cutting costs, there is also a need to use a less expensive fine powder.